The present invention relates in general to electronic circuits and, more particularly, to switching power supplies which converts an AC signal to a regulated DC signal.
Most switching power supplies have a transformer with a power switching transistor coupled to one side of the transformers primary winding. The power transistor turns on and off as determined by a regulator circuit to alternately store energy in the magnetic field of the transformer and transfer the stored energy to the secondary winding. The secondary winding of the transformer develops a DC output voltage across a shunt capacitor coupled across the secondary winding as a function of the energy transfer.
For example, FIG. 1 illustrates prior art switching power supply 10 that receives an AC line voltage and converts it to a DC bulk voltage by the full-wave rectifier diode bridge 12. Capacitor 14 filters the DC bulk voltage, and the primary winding of transformer 16 receives the DC bulk voltage. Regulator 18 modulates an inductor current through the primary winding of transformer 16 to control the amount of energy stored in the magnetic field of transformer 16. The energy stored in the magnetic field is transferred to the secondary winding where capacitor 20 is coupled across the secondary winding to develop the DC output voltage (VOUT). Diode 22 and photo-detection transistor 24 act together to optically couple feedback information from reference and error amplifier 26 to regulator circuit 18 to regulate VOUT of switching power supply 10.
Secondary side sensing of the voltage and current at the output to the switching power supply in the prior art requires at a minimum, a voltage reference, additional op amps, an opto-coupler, as well as a secondary supply voltage which operates at current limit. When at current limit, the output voltage typically is reduced which results in a reduction of the output voltage to operate the constant current circuitry. Thus, it is desired to have a switching power supply circuit that works entirely from the primary side of the transformer to eliminate the need for additional components. Also, a need exist as well to eliminate the secondary side supply required to operate a switching power supply while in current limit. Furthermore, a need exists to have a switching power supply circuit that operates in a constant current mode, and a constant voltage mode at the output to the switching power supply. The invention disclosed herein will address the above problems.